1. Field of the Invention
The present invention relates to enhancement of beam quality, oscillation efficiency, and reliability of a slab solid state laser apparatus, and to enhancement of a performance of a laser machining apparatus by using the slab solid state laser apparatus.
2. Description of the Prior Art
FIGS. 89 and 90 show a solid state laser apparatus including an exciting system and a cooling system for a conventional laser medium disclosed in, for example, Japanese Patent Application No. 64-84680 in addition to a conventional solid state laser apparatus disclosed in, for example, Japanese Patent Application Laid-Open No. 63-188980.
In FIGS. 89 and 90, reference numeral 1 means a slab solid state laser medium (hereinafter referred to as slab) including a pair of opposing smooth surfaces 11, a pair of side surfaces 12, and a pair of end surfaces 13 serving as an entrance surface and an exit surface for a laser beam. The slab is made of, for example, Nd:YAG (Yttrium Aluminum Garnet) obtained by doping Nd. Reference numeral 5 means a supporter disposed on the slab side surface 12, 2 means a lamp to perform light excitation of the slab 1, and 21 is the excitation light.
Reference numeral 7 means a frame integrally containing the slab 1 and the supporter 5, and an opening portion 711 is provided in the frame 7 to extend over substantially an entire surface of the smooth surface 11 of the slab as shown in FIG. 91. Reference numeral 70 means a sealant to seal water 41 serving as a coolant for the slab, and the sealant 70 extends over an entirely peripheral length of a plane formed by the slab surface (the smooth surface) 11 and the supporters 5 disposed on the side surface as shown in FIG. 92. Reference numeral 3 means a condenser to condense the excitation light for irradiation of the slab 1, and 81 and 82 are housings containing the condensers 3. Reference numeral 40 means a partition plate to form a flow path 4 of the slab cooling water 41, and the partition plate 40 is transparent with respect to the excitation light.
A description will now be given of the operation.
The slab 1 absorbs the excitation light 21 emitted from the lamp 2 to form inverted population. Energy of the inverted population is derived externally to the medium as a laser beam 100 which is propagated between the pair of opposing smooth surfaces 11 of the slab in a zigzag fashion while repeating internally total reflection. However, 50% or more excitation light energy absorbed by the slab becomes thermal energy in the slab, and finally flows out of the slab into the coolant 41 which is filled to contact the opposing smooth surfaces 11 of the slab. At this time, as shown in FIG. 93, there are generated the square temperature distribution having a hot center portion, and the square refractive index distribution along with the square temperature distribution in a thickness direction of the slab. The laser beam 100 in the slab, however, follows a zigzag optical path so that an effect of the square refractive index distribution can be canceled, and no laser beam is distorted. The coolant 41 is sealed by the sealants 70 on the opposing smooth surfaces 11 of the slab and the supporters 5 disposed on the side surfaces so as not to be externally leaked. The sealant 70 also serves as a supporter of the slab 1 to the frame 7.
In an ideal condition of slab laser, a laser medium can be uniformly excited in a width direction, and heat can also be uniformly generated in the width direction. Further, the slab can uniformly be cooled from only the slab surfaces (the opposing smooth surfaces) 11, one-dimensional square temperature distribution can be established in the thickness direction, and a uniform temperature distribution can be established in the width direction. For purpose of the ideal condition, the condenser 3 is employed such that the slab 1 can be irradiated with the most uniform and the most efficient excitation light 21 possible. For cooling, the coolant 41 uniformly flows on the slab surface 11, and the supporters 5 are adhered on the side surfaces 12 and are made of glass (having thermal conductivity K of 0.012 W/cm.sup.2 deg), fluorocarbon resin (having thermal conductivity K of 0.0025 W/cm.sup.2 deg), or silicon rubber (having thermal conductivity K of 0.0015 W/cm.sup.2 deg) which has a higher heat insulation property than that of the laser medium (i.e., YAG having thermal conductivity K of 0.12 W/cm.sup.2 deg).
A conventional laser oscillator is constructed as set forth above. There are problems in that heat insulating materials 5 on the slab side surfaces 12 and adhesives absorb the excitation light to generate heat, and generate a temperature distribution having hot side surfaces in a width direction and a concave lens-like optical distortion along with the hot temperature distribution on the side surface as shown in FIG. 94, resulting in degradation of beam quality and laser output. Further, there is another problem in that the beam quality varies according to an output level since the optical distortion depends upon excitation intensity.
In case reflectance of the supporter 5 with respect to the excitation light 21 is low, the excitation light 21 passes from the slab side surfaces 12 to the supporters 5 as shown in FIG. 95. Hence, there are other drawbacks in that a sufficient oscillation effect can not be provided, and uniform excitation can not be achieved due to degraded excitation intensity in the vicinity of the side surface.
When the slab 1 and the supporters 5 disposed on side surfaces thereof are contained in the frame 7, pressure caused by O-rings 70 in upper and lower surfaces apply force in a direction 510 to separate the slab 1 from the supporters 5 as shown in FIGS. 96, 97. Consequently, the coolant 41 can not be sufficiently sealed due to reduced adhesive properties between the slab side surfaces 12 and the supporters 5 so that water leakage occurs on the slab end surface 13. Thus, there are serious problems in that a beam may be cut away, and contamination may occur on the beam entrance/exit end surface 13 which is optically polished.
Further, in case the slab is strongly excited, mechanical deformation is generated in the vicinity of the slab side surfaces and in the vicinity of the entrance/exit end as shown in FIGS. 98, 99. As a result, there are problems in that the water leakage may occur due to reduced water-tightness between the slab side surfaces 12 and the supporters 5, the optical distortion may occur due to stress concentration 125 since the supporters 5 can contact the slab 1 with a pin point contact, and the slab 1 may be damaged if the worst happens.
Alternatively, substances having high reflectance may be employed as the supporter 5 in order to provide more enhanced oscillation efficiency. Most substances of this kind, however, have extremely poor adhesive properties, and it is typically difficult to provide compatibility of the heat insulating properties with the water-tightness for the slab side surface 12 even if the substances are used.